In order to provide an antireflection arrangement for surfaces, in particular for optical elements or displays, use is usually made of reflection-reducing interference layer systems containing a plurality of alternating layers of high refractive index and low refractive index materials. At the present time, MgF2 where n=1.38 is used as material having a particularly low refractive index in the visible spectral range. The antireflection effect of conventional dielectric layer systems could be improved if materials having a lower refractive index were available.
An alternative possibility for reducing the reflection of an optical element is known from the German patent specification DE 10241708 B4, also published as U.S. Patent Publication No. 2005/0233083 A1. In this method, a nanostructure is produced at the surface of a plastics substrate by means of a plasma etching process, the reflection of the plastics substrate being reduced by said nanostructure. Providing an optical element with an antireflection arrangement by producing a nanostructure at the surface of said optical element has the advantage of achieving low reflection over a wide angle-of-incidence range.
The German patent document DE 102008018866 A1, also published as U.S. Pat. No. 8,192,639 B2, describes a reflection-reducing interference layer system to which an organic layer is applied, said organic layer being provided with a nanostructure by means of a plasma etching process.
However, plasma-etched nanostructures attain only a depth of 100 nm to 200 nm on most materials. Such a thickness, for planar and slightly curved surfaces, suffices to provide an antireflection arrangement for a substrate in the visual spectral range of 400 nm to 700 nm for angles of light incidence of 0° (perpendicular light incidence) to 60° in such a way that the residual reflection is only approximately 1%. In some instances, however, there is a demand for broadband antireflection arrangements which are intended to function over even greater ranges of angles of light incidence.
Producing an antireflection layer on low refractive index (n<1.7) surfaces that are greatly curved poses a particular problem. A layer deposited by a directional vacuum coating process such as sputtering or vapor deposition has, at the location at which it grows, a thickness that is dependent on the angle of the impinging vapor. The layer thickness decreases as the incidence angle increases. In the case of an interference layer system, therefore, the physical thickness d of all the layers decreases as the incidence angle increases. However, the optical thickness n*d, where n is the refractive index, is of importance for the optical function. The refractive index n varies in the layer systems consisting of high refractive index and low refractive index materials, with the result that the optical function additionally varies with varying thickness. On account of this problem, the residual reflection of antireflection layers in the edge region of lenses generally has undesirably high values.
An improvement could be achieved if a low refractive index gradient layer could be produced with a thickness such that a decrease in thickness of at least 50% is tolerated. The technical realization on high refractive index substrates (n>1.7) proves to be simpler than on the conventional low refractive index glasses since a layer construction in which the refractive index gradually decreases can be realized even with natural materials.
There are technically only a small number of possibilities for producing relatively thick layers having an effective refractive index of <1.38. The document W. Joo, H. J. Kim and J. K. Kim, “Broadband Antireflection Coating Covering from Visible to Near Infrared Wavelengths by Using Multilayered Nanoporous Block Copolymer Films”, Langmuir 26(7), 2010, 5110-5114, describes the production of a thick gradient layer by means of sol-gel processes, but deposition on curved surfaces could be difficult here.
A method for producing multilayered gradient layers using vacuum technology is known from the document S. R. Kennedy, M. J. Brett, “Porous Broadband Antireflection Coating by Glancing Angle Deposition”, Appl Opt. 42, 4573-4579, 2003. In that case, oxides or fluorides are vapor-deposited onto the substrate at an oblique angle. Porous layers likewise arise here as a result of shading effects. For this reason, therefore, the substrate has to be positioned obliquely with respect to the angle of vapor incidence. On a greatly curved surface, however, additional shading effects would occur as a result of lens geometry, and so the method cannot readily be employed for curved lenses.
The International Patent Publication document WO 2014/202375 A1, also published as U.S. Pat. No. 9,039,906 B2 describes a reflection-reducing layer system comprising two nanostructured layers arranged one above the other. A broadband antireflection arrangement is achieved as a result, although scattered light losses can possibly arise as a result of the inner nanostructured layer.